In recent years, improvement of various properties is required for a rubber composition used for an automobile tire. Especially, improvement of mechanical properties, abrasion resistance and appropriately reduced rolling resistance (i.e., low heat-build up) is required for a rubber composition used for sidewall and bead of an automobile tire.
Natural rubber is consumed in a large amount for an automobile tire. In many cases, other rubbers are blended with natural rubber for improving various properties of natural rubber. In one example, a polybutadiene rubber is blended for improving the abrasion resistance, and, in another example, a styrene-butadiene copolymer rubber is blended for improving the mechanical properties. However, there is a basic contradiction between properties required for a tire rubber, and thus, all of the properties required for an automobile tire are difficult to satisfy concurrently. For example, enhancement of abrasion resistance is accompanied by mechanical properties, and enhancement of mechanical properties is accompanied by increase of heat-build up.
In view of miscibility of a rubber raw material with a reinforcing agent or other additives, it is usually desired that a rubber raw material does not have a gel structure. However, a rubber gel having a gel structure has been proposed for giving a rubber having low heat-build up and high abrasion resistance. For example, a rubber composition comprising a polychloroprene gel was proposed in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. H3-37246. This rubber composition gives a rubber exhibiting satisfactory low heat-build up and high abrasion resistance, but, as the polychloroprene gel contains chlorine, it causes a problem of damaging the environment when scrap tires are incinerated, which makes it's practical use difficult.
Further, as for a conjugated diene rubber, a rubber composition comprising a polybutadiene gel, and a rubber composition comprising a styrene-butadiene copolymer rubber gel were proposed in JP-A H6-57038 and JP-A H10-204217, respectively. These rubber compositions give rubbers having satisfactory low heat-build up, but the resulting rubbers occasionally exhibit poor abrasion resistance, reduced breaking elongation and poor mechanical properties.
An emulsion polymerization procedure is widely employed for the production of a conjugated diene rubber. In the production process by an emulsion polymerization procedure, a latex prepared by emulsion polymerization in a manner such that a desired polymer composition is obtained is coagulated with an inorganic salt as a coagulant to form crumbs having a size of about 2 to 10 mm, water is separated therefrom, and the crumbs are washed and then dried to give an object rubber. However, for example, in the case where a butadiene-styrene copolymer rubber having a high, e.g. about 35 to 50% by weight of, bound styrene content is produced by an emulsion polymerization procedure, a latex as obtained by emulsion polymerization exhibits a poor coagulability, and therefore, there is a tendency such that the latex is partly non-coagulated and the thus-formed sticky crumbs are bonded together to form unusually large crumbs. Such large crumbs have a problem such that water tends to remain inside of each crumb and thus, a substantially long period is required for drying and partial drying failure occurs to form a wet spot. Further, the sticky crumbs are liable to be deposited on an inner wall of a coagulating vessel or on stirrer vanes.
To solve the above-mentioned problems caused by the sticky crumbs, procedures for appropriately adjusting the concentration of an inorganic salt coagulant, the solid content in the latex, the coagulation temperature and the stirring conditions, or for using a polymeric flocculant or a heat-sensitive coagulant in addition to the inorganic salt coagulant. However, the above-mentioned problems are difficult or even impossible to completely solve by these procedures.